Waterline

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The waterline is the line where a ship’s hull meets the water when the boat is level. It’s also used for any line on a hull that runs parallel to the water’s surface in a level position. Waterlines are part of naval architecture drawings that describe the hull’s shape.

The load line, or Plimsoll line, is a safety mark on ships that shows how much weight they can carry in different water types and temperatures to stay buoyant and safe.

For ships with displacement hulls, the length of the hull at the waterline helps determine the hull’s speed. On sailing boats, the waterline length can change as the boat heels, which can affect speed.

In aircraft, the term waterline has a different meaning: it is a horizontal reference line from the aircraft’s nose to its tail. The base line is called waterline 0, and other waterlines are parallel to it. Measurements are usually in inches and increase as you move up from the base line. The exact location of the base line can vary by aircraft, sometimes being the tip of the nose or a standard ground plane.